This invention pertains to inkjet inks, in particular to inkjet inks comprising self-dispersing pigment formulated for improved optical density.
Inkjet recording is a printing method wherein droplets of ink are ejected through fine nozzles to form letters or figures on the surface of recording media. Inks used in such recording are subject to rigorous demands including, for example, good dispersion stability, ejection stability, and good fixation to media.
Both dyes and pigments have been used as colorants for inkjet inks. While dyes typically offer superior color properties compared to pigments, they tend to fade quickly and are more prone to rub off. Inks comprising pigments dispersed in aqueous media are advantageously superior to inks using water-soluble dyes in water-fastness and light-fastness of printed images.
Pigments suitable for aqueous inkjet inks are in general well-known in the art. Typically, pigments are stabilized by dispersing agents, such as polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the vehicle. More recently, so-called “self-dispersible” or “self-dispersing” pigments (hereafter “SDP(s)”) have been developed. As the name would imply, SDPs are dispersible in water without dispersants.
SDPs are often advantageous over traditional dispersant-stabilized pigments from the standpoint of greater stability and lower viscosity at the same pigment loading. This can provide greater formulation latitude in final ink. SDPs, and particularly self-dispersing carbon black pigments, are disclosed in, for example, U.S. Pat. Nos. 2,439,442, 3,023,118, 3,279,935 and 3,347,632. Additional disclosures of SDPs, methods of making SDPs and/or aqueous ink jet inks formulated with SDP's can be found in, for example, U.S. Pat. Nos. 5,554,739, 5,571,311, 5,609,671, 5,672,198, 5,698,016, 5,707,432, 5,718,746, 5,747,562, 5,749,950, 5,803,959, 5,837,045, 5,846,307, 5,851,280, 5,861,447, 5,885,335, 5,895,522, 5,922,118, 5,928,419, 5,976,233, 6,057,384, 6,099,632, 6,123,759, 6,153,001, 6,221,141, 6,221,142, 6,221,143, 6,277,183, 6,281,267, 6,329,446, 6,332,919, 6,375,317, US2001/0035110, EP-A-1086997, EP-A-1114851, EP-A-1158030, EP-A-1167471, EP-A-1122286, WO01/10963, WO01/25340 and WO01/94476.
All of the preceding disclosures are incorporated by reference herein for all purposes as if fully set forth.
In general, the STPs are obtained by reaction of pigments. These reactions often lead to anionic or cationic species on the surface of the pigment. In the case of anionic species such as carboxyl groups, the charge of the anionic group is balanced by a cation. Normally, this cation charge comes from monovalent cations such as sodium, potassium or lithium.
One way to take advantage of the latitude afforded by SDPs is to load more pigment into the ink formulation to increase optical density (OD). However, it would be even more advantageous to achieve high optical density without increasing the level of colorant.
Previously incorporated U.S. Pat. No. 6,332,919 and EP-A-1086997 disclose a black inkjet ink comprising an SDP and salts of monovalent cations. It is suggested that the presence of these monovalent salts improves optical density at a given pigment loading.
Previously incorporated U.S. Pat. No. 6,277,183 discloses a black inkjet ink comprising an SDP ink and a metal oxide, where optical density of the ink is higher when metal oxide is present than when it is absent.
Previously incorporated U.S. Pat. No. 6,153,001 discloses an example of a black inkjet ink containing an SDP (Microjet® CW1) and 9 ppm calcium. No information is provided on the source or physical state of the calcium or on the nature of the SDP. No suggestion is made of any optical density relationship.
Previously incorporated U.S. Pat. No. 6,375,317 discloses an inkjet ink comprising an SDP and calcium in an aqueous medium. In the only example in such disclosure, the SDP is functionalized with -phenyl-COONa groups, but no indication of degree of treatment (functionality) is provided. These types of SDPs, however, are typically of higher functionality. In addition, in this example, only about 2 ppm of Ca(OH)2 is used (about 1.2 ppm Ca). The calcium is said to be added to improve the ejection stability of the inks, and no suggestion is made of any optical density relationship connected to the addition of calcium.
It is an object of this invention to provided inkjet inks, in particular SDP-containing inkjet inks, with increased optical density, and to provide methods for increasing the optical density and/or stability of such inks.